1. Field of the Invention
The present invention relates to an image forming apparatus such as printers and copying machines based on electrophotography.
2. Description of the Related Art
An electrophotographic image forming apparatus involves charging, developing, transferring, and fixing processes. A charging unit charges the surface of a photoconductive drum uniformly. An exposing unit illuminates the charged surface of the photoconductive drum in accordance with image data to form an electrostatic latent image. A developing unit supplies toner to the electrostatic latent image to develop the electrostatic latent image into a toner image. The toner image is then transferred onto a print medium such as print paper. Then, the print medium advances to a fixing unit where the toner image is fused into a permanent image. After fixing, the print medium is discharged onto a stacker.
A developing roller held in the developing unit includes a resilient layer of semi-conductive urethane rubber. The surface of the urethane rubber is formed by dipping the developing roller in a chemical solution or by coating with a chemical solution. Subsequently, the developing roller is heated to increase the ability to be triboelectrically charged, decrease the friction coefficient of the developing roller in contact with a toner supplying roller, and prevent contamination of the photoconductive drum.
An image pattern is often printed which has a partially high print duty such as a ruled pattern extending in a sub-scanning direction perpendicular to a direction of travel of the print medium. Continuous printing of such an image pattern causes the areas on the surface layer of the developing roller subjected to the high print duty to wear out. The wear of the developing roller causes the diameter of the developing roller to decrease, thereby decreasing a nip formed between the developing roller and the photoconductive drum. This leads to partially vague images or deposition of toner charges to an unwanted polarity.